The present invention relates to radar systems and methods for directionally selective detection of objects, for measurement of range and range rate of objects, and for control of radar carrying vehicles responsive to the detection of objects and their range and range rates relative to the vehicle. More particularly, one embodiment of the present invention relates to a radar apparatus for detecting objects in a detection cone. This directionally selective radar apparatus may be employed in providing collision avoidance control for a vehicle carrying the radar apparatus. The system may also employ a novel signal processing circuit for extracting range information from a received periodic rectangular-wave frequency-shift modulated carrier wave.
Two types of directional radar are known in the prior art: phased array radar systems and systems employing antenna polar patterns to achieve directionality.
Phased array radar systems tend to be complex and expensive. The more common type of directional radar for achieving spatial selectivity relies on the use of larger and larger antenna apertures to achieve directionality in the intensity of broadcast or in the sensitivity of reception of the radar signal. In many applications this approach has disadvantages relating to considerations of antenna size, and avaliability of space, system cost, weight, aesthetics, wind load factors, etc.
Accordingly, it is an object of the present invention to provide an inexpensive and easily fabricated directionally selective radar system.
A number of radar techniques for determining range are known in the prior art. Among these techniques are pulse radar, which measures the transit time of a reflected pulse, and continuous wave radar, which, in typical embodiments, derives distance information from phase shifts in one or more continuous carrier waves of constant frequency. Both systems have the disadvantage that they are expensive and difficult to implement since each requires complex signal processing. Moreover, the expense of these systems increases in inverse relation to the distances being measured.
A known radar technique for determining range employs a rectangular frequency-shift modulated carrier wave, i.e., a carrier wave whose frequency changes abruptly at periodic intervals between a higher and a lower frequency. In the known system, a broadcast rectangular-wave frequency-shift modulated carrier wave is mixed with its returned replica reflected by an object of interest. If the object and radar system are moving relative to one another, higher and lower frequency portions of the signal will each exhibit a Doppler shift. These Doppler shifts are separated, filtered and compared with respect to phase in order to extract range information from the phase difference. The system has at least two disadvantages: the aforementioned signal processing is generally complex and expensive to implement and the system cannot detect the range of an object which is moving with a small or zero velocity with respect to the radar system. This is so because the Doppler shifts in such situations are negligible.
Accordingly, it is another object of the present invention to provide an inexpensive and easily fabricated short range radar system.
It is yet another object of the present invention to provide an inexpensive and easily fabricated radar system employing frequency-shift modulated carrier waves.
It is yet another object of the present invention to provide a radar system employing frequency-shift modulated carrier waves for detecting objects with a zero or small relative velocity with respect to the radar system.
The aforementioned features of the present invention viz. economy, directional selectivity, compactness, and short range capability, render it well suited for use in vehicles, and particularly well suited for use in collision avoidance systems for automobiles.
There exists a pressing need for a reliable system to eliminate or minimize the property damage and personal injury associated with vehicle collisions.
Vehicle control systems are known in the prior art which employ light, radio or sonic waves to detect objects. Many of these systems are not self-contained but require that special active or passive components be used on the detected objects--other automobiles, guard rails, etc. This is a significant disadvantage in that it increases the expense of the system and requires the cooperation of other motorists, local governments, etc., so that components external to the controlled vehicle are provided.
Accordingly, it is yet another object of the present invention to provide a practical self-contained vehicle radar system.
A self-contained vehicle radar system is known in the prior art, which employs the prior art rectangular-wave frequency-shift carrier wave detection technique discussed above. However, the system, as noted above, is expensive and incapable of detecting objects moving at nearly the same velocity as the controlled vehicle and is not sufficiently directionally selective unless a relatively large directional antenna is employed. These disadvantages present particular difficulty in vehicle collision avoidance systems for a number of reasons. First, the collision avoidance system must be sufficiently inexpensive so that it may be widely used. Second, the system must be capable of detecting objects at rest with respect to the controlled vehicle, since, for example, collision avoidance may require response to unsafe following distances. Third, the system must be directionally selective so that it has the capacity to discriminate collision threats from normal traffic situations which present no immediate threat.
Accordingly, it is yet another object of the present invention to provide an economical and easily implemented vehicle radar system which is capable of detecting objects with small or zero velocity with respect to a radar equiped vehicle.
It is yet another object of the present invention to provide an easily implemented directionally selective vehicle radar system.
It is yet another object of the present invention to provide an inexpensive and easily implemented collision avoidance system for a vehicle.
The use of radar in moving vehicles for measuring speed and distance relative to road obstacles has thus been discouraged by the inability of prior art methods of implementation to provide reliable discrimination against surrounding reflectors or scatterers (such as overpasses, overhanging road signs, objects in side lanes or on the road side, center dividing fences, side railings, etc.) of the probing radar signal. Attempts to effect such discrimination have thus been mainly based on using more highly directive antenna polar (radiation and receiving) patterns and/or special passive reflectors or active transponder mounted on the tail ends of other vehicles.
Highly directive antenna patterns require larger antenna apertures, which causes problems of installation, obstruction of the radiator, aerodynamic compatibility, and aesthetics or styling. Alternatively, phased-array techniques for effecting spatial selectivity are generally complex and costly. Resort to operating frequencies in the 15 GHz to 40 GHz range to reduce the physical size of high gain antennas introduces not only the risk of excessive signal absorption by fog, rain and snow, but also pushes the RF power source and related processing requirements into a technology that is still in the developmental stage from the viewpoint of mass-producible, low-cost components.
Accordingly, it is yet another object of the present invention to provide an easily implemented collision avoidance system capable of discriminating collision threats from safe vehicle environments.
These and other objects and features of the invention will become apparent from the claims, and from the following description when read in conjunction with the accompnaying drawings.